Fluidised bed incinerators

ABSTRACT

To raise or lower the operating temperature of a fluidised bed to a desired level, the operating temperature of a second fluidised bed is raised or lowered to a temperature above or below that of the desired level and bed material is transferred between the two beds to bring the operating temperature of the first bed to the desired level. The first bed may be an incinerator and to lower the operating temperature the bed material of the second bed is cooled by a fluid in a heat exchanger and material is transferred between the two beds. The transfer of bed material may be brought about by a pump or one bed may be caused to overflow into the other with a corresponding counter-flow of bed material through a duct connecting the two beds.

'tlnited States Patent 1191 Mitchell et a1.

1451 Nov. 25, 1975 FLUIlDlSED BED INCINERATORS [22] Filed: Oct. 19, 1973[21] Appl. No.: 407,773

[30] Foreign Application Priority Data Oct. 20, 1972 United Kingdom48339/72 [56] References Cited UNITED STATES PATENTS 1/1964 Sweeney110/28 X 2/1967 Campbell 2/1970 Jacuboweiz 122/4 X 3,524,633 8/1970Shiller 110/8 X 3,625,165 12/ l 971 lshigaki 3,636,896 1/1972 McLaren eta1, 110/8 Primary Examiner-Kenneth W. Sprague Attorney, Agent, orFirm-Brisebois & Kruger [57] ABSTRACT To raise or lower the operatingtemperature of a fluidised bed to a desired level, the operatingtemperature of a second fluidised bed is raised or lowered to atemperature above or below that of the desired level and bed material istransferred between the two beds to bring the operating temperature ofthe first bed to the desired level. The first bed may be an incineratorand to lower the operating temperature the bed material of the secondbed is cooled by a fluid in a heat exchanger and material is transferredbetween the two beds. The transfer of bed material may be brought aboutby a pump or one bed may be caused to overflow into the other with acorresponding counter-flow of bed material through a duct connecting thetwo beds.

4 Claims, 4 Drawing Figures US, Patent Nov.25, 1975 Sheet10f2 3,921,590

U.S. Patent Nov. 25, 1975 Sheet 2 of2 3,921,590

FLUIDISED BED INCINERATORS This invention relates to a method of andapparatus for raising or lowering the operating temperature of afluidised bed.

Fluidised beds are well known and may be employed to raise or lower thetemperature of articles supported in the bed and they may also be usedas incinerators by burning combustible material in the bed. When afluidised bed is used to heat treat material such as metal castings itis necessary to keep the operating temperature of the bed at asubstantially constant value. Alternatively if the bed is being used tocool articles supported in it then again it is necessary to control theoperating temperature of the bed. Furthermore if the fluidised bed isused as an incinerator and the calorific value of the material beingburnt in the bed is high then the operating temperature of the bed mayrise to an unacceptable level in which case the temperature of the bedhas to be reduced. If a heat exchanger is located in the bed so that bypassing fluid of the appropriate temperature through the heat exchangerthe temperature of the bed can be controlled, it has been found that thetubes of the heat exchanger may become partially covered with bedmaterial thereby reducing the heat transfer from the bed material to thefluid in the tubes and this situation can also lead to pockets ofdefluidisation of the bed. When the bed is used as an incinerator thetubes of the heat exchanger are subject to a high corrosion risk due tothe hot wet products of combustion and also to erosion effects due tothe fluidised bed particles.

It is an object of the present invention to provide a method of raisingor lowering the operating temperature of a fluidised bed in which thesedifficulties are overcome.

It is a further object of the present invention to provide fluidised bedapparatus in which the abovementioned difficulties are overcome.

According to a first aspect of the present invention in a method ofraising or lowering the operating temperature of a first fluidised bedof refractory particulate material to a desired level, the operatingtemperature of a second fluidised bed of refractory particulate materialis raised or lowered to a temperature above or below that of the desiredlevel of the first bed respectively and sufficient bed material istransferred between the two beds to raise or lower the operatingtemperature of the material of the first bed to said desired level.

According to a second aspect of the invention fluidised bed apparatuscomprises first and second beds of refractory particulate material,separate means for fluidising the two beds, means associated with thesecond bed for raising or lowering the operating temperature of the bedand means for transferring bed material between the two beds.

In order that the invention may be more readily understood it will nowbe described, by way of example only, with reference to the accompanyingdrawings in which:

FIG. 1 is a diagrammatic side elevation of fluidised bed apparatus inaccordance with one embodiment of the invention,

FIG. 2 is a diagrammatic side elevation of a fluidised bed incineratorin accordance with one embodiment of the invention.

FIG. 3 is a diagrammatic side elevation of a fluidised bed incineratorin accordance with the present invention, the bed having internalcirculation of the bed material, and

FIG. 4 is a diagrammatic side elevation of a fluidised bed incineratorin accordance: with a still further embodiment of the invention.

Referring to FIG. 1, fluidised bed apparatus consists of two vessels 1,2 each containing a bed 3 of refractory particulate material such assand. In each vessel the bed material is supported on a gas distributor4. The distributors may take the form of apertured plates or porousmaterial or any other form of distributor suitable for use with afluidised bed. Beneath each distributor there is a wind box 5 and gas,conveniently air, is supplied under pressure to each of the boxes 5through a pipe 6. The two vessels are interconnected by a duct 7extending between the vessels at a level above the distributors 4 andthe duct contains pumping means 8 which may take the form of a vane orother suitable type of pump. When the apparatus is in use and the bedsare fluidised the normal surfaces of the beds are indicated by referencenumeral 9. Above the level of the surfaces 9 the two vessels 1, 2 areinterconnected by a further duct 10. Located in the vessel 2 is a heatexchanger 11 which is in heat transfer relation with the bed material 3when the material in the vessel 2 is fluidised.

If the fluidised bed in vessel 1 is to be used to heat articles 12 suchas metal castings to a desired level then fluid is circulated throughthe heat exchanger 11 to raise the temperature of the material in thebed 2 to a higher level than the desired temperature of the material inthe vessel 1. Sufficient bed material is transferred from vessel 2 intothe vessel 1 to raise the temperature of the bed material in the vessel1 to the desired level. At the same time an equal quantity of bedmaterial from the vessel 1 is transferred to vessel 2. In this way theoperating temperature of the bed material in vessel 1 can be raised tothe desired level. The bed material is transferred for example byoperating the pumping means 8 to pump the fluidised bed material fromvessel 1 into vessel 2 thereby raising the level of the surface of thematerial in vessel 2 so that bed material flows from vessel 2 along theduct l0 and into the vessel 1. Obviously the pumping means could beoperated in the opposite direction so that bed material is pumped fromvessel 2 to vessel 1 through the duct 7 causing the level of the surfacein vessel 1 to rise and for bed material to flow through the duct 10into vessel 2.

If it is desired to cool the articles 12 suspended in the vessel 1 thenthe operating temperature of the first bed has to be reduced so acoolant fluid is passed through the heat exchanger 11 in vessel 2thereby reducing the temperature of the bed material in that vessel tobelow the desired operating temperature of the bed in vessel 1. Hot bedmaterial from vessel 1 is then transferred to vessel 2 and cooler bedmaterial from vessel 2 is transferred to vessel 1 thus lowering theoperating temperature of the bed material in vessel 1.

Referring now to the embodiments of the invention shown in FIGS. 2, 3and 4 where the invention is applied to a fluidised bed incinerator, avessel 21 is lined with solid refractory material 22 and has an internalpartition 23 dividing the space within the vessel into two side-by-sidecompartments 24 and 25 respectively. The base of each compartmentcomprises an apertured plate 26 and a bed of refractory particulatematerial 27 is contained in each compartment and supported on the base26. A flue 28 extends out of the vessel from the compartment 24 andwaste material such as town refuse is introduced into the compartment 24by way of a hopper 29 and a screw feeder 30. At the base of thecompartment 24 there is an outlet 31 through which the ashes ofcombustion and non-combustible content of the waste material can beperiodically removed. A heat exchanger in the form of a bundle of boilertubes 32 is positioned in the compartment so as to be in heat transferrelation with the bed material in that compartment when the material isfluidised.

In use, air under pressure is supplied below the apertured plates 26 inorder to fluidise the beds to particulate material in the compartments24 and 25 to a state beyond that of teter. A quantity of waste materialand gas or fuel oil if necessary are introduced into compartment 24 tostart combustion of the waste material. As soon as the bed has reached atemperature high enough to promote spontaneous combustion of the wastematerial supplied to it the gas or fuel oil is cut off.

Referring now to FIG. 2, the partition 23 extends from a position abovethe bases of the beds and above the normal operating surfaces of thefluidised beds. A small amount of bed material may pass between the twocompartments but the bed material in compartment 24 will be considerablyhotter than that in compartment 25. When the temperature of the bedmaterial in compartment 24 approaches its desired operating level, thetransfer of bed material between the two compartments is increased byincreasing the degree of fluidisation of the bed in compartment 25 byincreasing the air supplied to it thereby causing the bed to expand andfor some of the bed material to spill over from compartment 25 intocompartment 24. At the same time hot bed material from compartment 24flows beneath the partition 23 into compartment 25 where it gives upmost of its excess heat to cooling fluid circulated in the heatexchanger 32. In this way, hot bed material flows from compartment 24 tocompartment 25 and cooler bed material flows from compartment 25 to 24thus reducing the temperature of the bed material in compartment 24. Thetransfer of bed material may be carried out continuously or it may beoperated intermittently so as to reduce the bed temperature incompartment 24 to a level below the desired operating temperature andthen allowing the operating temperature to rise to approaching themaximum desired level before bed transfer takes place again.

Referring to the embodiment of the invention shown in FIG. 3 theapertured plate 26 in the compartment 24 is inclined downwardly towardsthe plate 26 in the compartment 25 and the space beneath the inclinedplate 26 is divided into a plurality of sections 33, 34 and 35 arrangedside-by-side and each having its own air supply. In use, air atdifferent mass flows is supplied to the sections 33, 34, 35 with thesupply to section 35 being at a higher mass flow than that supplied tosection 34 which in turn is at a higher mass flow than that supplied tosection 33. This differential air mass flow causes the bed material incompartment 24 to be in a state of agitation and to circulatecontinuously as indicated by the arrow 36. Waste material introducedinto the compartment by the screw feeder is quickly enveloped by thecirculating bed material and is drawn down to the base of the bed. Thecombustible content of the material is burnt in the bed and thenoncombustible content is displaced to the outlet 31. The circulationofthe bed material in compartment 24 is in a vertical plane and in adirection downwardly towards the base of the partition 23. Thiscirculation of the bed material influences the bed material incompartment 25 and hot bed material is continuously transferred intocompartment 25 and cooler material spills over the top of the partition23 into the compartment 24. A deflector plate 37 is positioned at thebase of the flue 28 to prevent bed material from being drawn into theflue as it is being transferred from one compartment to the other.

Referring to the embodiment of the invention illustrated in FIG. 4, ahigh pressure air jet is employed to assist and control transfer of thebed material from compartment 24 to 25. The apertured plate 26 in thecompartment 25 is located at a level above the base of the partition 23and a duct 38 extends from compartment 24 beneath the partition 23 andupwardly to a level above the plate 26 in compartment 25. A cover isfitted above the upper end of the duct 38 to reduce the flow of bedmaterial into the duct from compartment 25 and an injector pipe 39extends upwardly through the duct 38 and terminates close to the base 26in the compartment 25.

In use, the bed material in compartment 24 is caused to circulate in avertical direction due to the differential air mass flow supplied to thecompartment. Some of the bed material will enter into the duct 38. Whenit is desired to transfer bed material between the two compartments, airat a high pressure is supplied to the injector pipe 39 and the airentrains hot bed material in the duct 38 and transfers it to above theplate 26 in the compartment 25. A corresponding amount of cooler bedmaterial is forced out of the compartment 25 over the partition 23 andinto compartment 24. The pressure of air supplied to the injector pipe39 and hence the quantity of bed material transferred between the twocompartments is controlled according to the temperature of the bedmaterial in compartment 24.

In the embodiments of the invention where the beds of refractoryparticulate material are shown supported on apertured plates 26, theplates serving as distributors for the fluidising medium, an alternativeto an apertured plate is a porous plate or any other form ofdistributor. Where the plate 26 of the compartment 24 is shown withthree air boxes beneath it, it is to be understood that two or more airboxes may be employed or a single box may be employed if the airdistributor is so arranged that the permeability/porosity of thedistributor varies along its length to allow differing mass flowsthrough it along its length.

In other embodiments of the invention various types of pumping devicesmay be used for pumping bed material from one compartment to another.For example mechanical or electro magnetic pumps may be used and thepumps may be cooled by positioning them in a duct through which airunder pressure is passed.

In a still further embodiment of the invention the two compartments areseparated by a partition which is porous and by vibrating the partition,bed material can be caused to pass from one compartment to the other.

In all the embodiments of the invention where a boiler tube type heatexchanger is shown it is to be understood that other types of heatexchanger may be employed.

In the embodiments of the invention which relate to a fluidised bedincinerator. the transfer of bed material between the two compartmentsserves two useful purposes. The replacement of hot bed material bycooler bed material in the incinerator reduces the operating temperatureof the incinerator and thus avoids the necessity of introducingquenching water or air attemperation in the incinerator and much of theheat contained in the hot bed material is given up to the fluid in theheat exchanger, the fluid being used for power generation or otherprocess requirements. As the heat exchanger is not contained in the partof the fluidised bed in which combustion takes place, corrosion of theheat exchanger due to hot products of combuation is eliminated.Furthermore, erosion of the heat exchanger is minimised by the use offluid mass flow rates lower than those obtaining in the main combustionfluidised bed.

We claim:

1. Apparatus comprising first and second beds of refractory particulatematerial, said beds being located in separate compartments arranged sideby side in a vessel, a partition forming a common wall between saidcompartments and extending from a position above the lowest level ofsaid particulate material to a position above the normal upper surfacesof said particulate material, separate means for fluidising the twobeds, the means for fluidising the first bed being arranged to fluidisethe bed in a non-uniform manner to cause a greater degree of agitationof the bed adjacent said partition than away from said partition,thereby promoting circulation of the material in said first bed from theupper surface of said first bed which is away from said partitiondownwardly toward the bottom of the partition, and means associated withthe second bed for raising or lowering the operating temperature of thesecond bed and for transferring material between the two beds.

2. Apparatus comprising first and second beds of refractory particulatematerial, said beds being located in separate compartments arranged sideby side in a vessel, separate means for fluidizing the two beds, apartition forming a common wall between said compartments andextending-from a position above the lowest level of said particulatematerial to a position above the normal upper surfaces of saidparticulate material, means associated with the second bed for raisingor lowering the operating temperature of the second bed, and means fortransferring bed material between the two beds which comprises a ductextending beneath the partition from a position above the base of thefirst bed to a position above the base of the second bed, and pumpingmeans in the duct arranged to transfer bed material through the ductfrom the first bed to the second bed.

3. Apparatus as claimed in claim 2 in which said pumping means comprisesa pneumatic injector.

4. Apparatus comprising first and second beds of refractory particulatematerial, said beds being located in separate compartments arranged sideby side in a vessel, separate means for fluidizing the two beds, apartition forming a common wall between said compartments and extendingfrom a position above the lowest level of said particulate material to aposition above the normal upper surfaces of said particulate material,means associated with the second bed for raising or lowering theoperating temperature of the second bed, and means for transferring bedmaterial between the beds, said vessel defining a duct extending beneaththe partition from a position above the base of the first bed to aposition above the base of the second bed and pumping means in the ductarranged to transfer bed material through the duct from the first bed tothe second bed.

1. APPARATUS COMPRISING FIRST AND SECOND BEDS OF REFRACTORY PARTICULATEMATERIAL, SAID BEDS BEING LOCATED IN SEPARATE COMPARTMENTS ARRANGED SIDEBY SIDE IN A VESSEL, A PARTITION FORMING A COMMON WALL BETWEEN SAIDCOMPARTMENTS AND EXTENDING FROM A POSITION ABOVE THE LOWEST LEVEL OFSAID PARTICULATE MATERIAL TO A POSITION ABOVE THE NORMAL UPPER SURFACESOF SAID PARTICULATE MATERIAL, SEPARATE MEANS FOR FLUIDISING THE TWOBEDS, THE MEANS FOR FLUIDISING THE FIRST BED BEING ARRANGED TO FLUIDISETHE BED IN A NON-UNIFORM MANNER TO CUASE A GREATER DEGREE OF AGITATIONOF THE BED ADJACENT SAID PARTITION THAN AWAY FROM SAID PARTITION,THEREBY PROMOTING CIRCULATION OF THE MATERIAL IN SAID FIRST BED FROM THEUPPER SURFACE OF SAID FIRST BED WHICH IS AWAY FROM SAID PARTITIONDOWNWARDLY TOWARD THE BOTTOM OF THE PARTITION, AND MEANS ASSOCIATED WITHTHE SECOND; BED FOR RAISING OR LOWERING THE OPERATING TEMPERATURE OF THESECOND BED AND FOR TRANSEFERRING MATERIAL BETWEEN THE TWO BEDS. 2.Apparatus comprising first and second beds of refractory particulatematerial, said beds being located in separate compartments arranged sideby side in a vessel, separate means for fluidizing the two beds, apartition forming a common wall between said compartments and extendingfrom a position above the lowest level of said particulate material to aposition above the normal upper surfaces of said particulate material,means associated with the second bed for raising or lowering theoperating temperature of the second bed, and means for transferring bedmaterial between the two beds which comprises a duct extending beneaththe partition from a position above the base of the first bed to aposition above the base of the second bed, and pumping means in the ductarranged to transfer bed material through the duct from the first bed tothe second bed.
 3. Apparatus as claimed in claim 2 in which said pumpingmeans comprises a pneumatic injector.
 4. Apparatus comprising first andsecond beds of refractory particulate material, said beds being locatedin separate compartments arranged side by side in a vessel, separatemeans for fluidizing the two beds, a partition forming a common wallbetween said compartments and extending from a position above the lowestlevel of said particulate material to a position above the normal uppersurfaces of said particulate material, means associated with the secondbed for raising or lowering the operating temperature of the second bed,and means for transferring bed material between the beds, said vesseldefining a duct extending beneath the partition from a position abovethe base of the first bed to a position above the base of the second bedand pumping means in the duct arranged to transfer bed material throughthe duct from the first bed to the second bed.